Metal-air batteries, such as zinc-air batteries offer the advantage of very high energy densities (up to 300 WHr/Kg) over known conventional batteries such as the lead-acid type employed in automotive vehicles. Zinc-air batteries can be manufactured on a commercial production basis at low cost and with a high degree of safety. However, commercial applications of zinc-air batteries have previously been limited to primary or non-rechargeable types. Experimental rechargeable zinc-air batteries have been built for use in automotive applications. These batteries use a liquid electrolyte and usually include a pump to recirculate the electrolyte. Such systems are impractical for miniature consumer applications ranging from radios to portable computers because of their mechanical complexity and lack of leak resistance.
A zinc-air battery generally includes a porous zinc anode, an air cathode formed of a carbon membrane, and a porous material containing a liquid electrolyte which is sandwiched between the anode and cathode. A reservoir of electrolyte is needed below the anode since, in practice, electrolyte becomes entrapped in the zinc matrix. As the zinc oxide expands, the electrolyte is forced upwardly. A major problem which exists in the development of rechargeable zinc-air batteries is that the oxygen gas which is generated by the anode during the charging cycle forces electrolyte accumulated above the anode upwardly into and through the air cathode. This results in loss of electrolyte due to evaporation, and contamination of electrolyte through reaction with the atmosphere and catalyst in the air cathode.